Process of heat-treating steel



Patented Jan. 6, 1931 PATENT orrlca FRANK A. FAHRENWALD, OF CHICAGO,ILLINOIS PROCESS OF HEAT-TREATING STEEL Application filed. June 25,

This invention relates to the heat treatment of steel strips, sheets,strands, rods, and wires and has for its object the provision of aprocess for producing on the surface of the steel at the very beginningof the heat treatment a thin, uniform, ent, shall shield the steel fromoxidation during the remaining steps of the process, and shall in alsoto a considerable degree protect the same from subsequent scaling andweather-oxidation, but without impeding subsequent operations on themetal such as drawing, pickling, painting, etc. Heat treating of somekind is 1:) very old, the earlier methods generally entailing'theheating and cooling of piled or stacked sheets, and the more recentmethods entailing the passing of the sheets through heating and coolingzones in a continuous moving web or rod. The old method exhibited thedefects that owing to the large mass to be heated and cooled the timesrequired were long, often 24 hours per cycle, resulting in abnormalgrain growth and non-uniformity of treatment as 25 between the interiorand exterior of the pile or pack. The next step was the development ofthe continuous furnace which discharged the steel into the open air atthe full furnace temperature, which saved time and graingrowth butentailed two new main difficulties,

first, that the action of the air on the hot steel produced excessivescaling, and second, the rapid cooling rate produced excessive hardness.The old process, however, exhibited one advantage which the newerprocess lacked,

namely that it was possible to produce unscaled sheets which the newprocess could not accomplish. Various attempts were made, such forexample as those set forth in the I Marsh and Cochran Patent No.1,610,567, to

produce bright annealed sheets but the difficulty of maintaining a truenon-oxidizing atmosphere is very great since ferrous metal at alltemperatures above-a red heat will become oxidized in contact withordinarily recognized as reducing, such as these produced by thecombustion of hydrocarbon fuel with deficiency of oxygen. lgurthermorethe rapidity with which the metal passes through the furnace requiresthat the furnace fine-grained, tenacious, adher-- impervious protectivecoating which" 1928. Serial no. 287,911.

be very long if the metal is to be cooledto a point where it can bedischarged into the air in a bright annealed condition, since itsdischarge at any temperature above about450 F. entails the formation ofloosely adherent oxides of the higher chemical valency such as Fe 0 orFe 0 The object of my invention is not to produce a bright annealedsheet, but rather to produce, in the early stages of the operation, asub-oxide coating of a nature which will not itself injure thesheet forany subsequent practical use but will itself, when treated as hereinafter described, protect the sheet from the formation of the looselyadherent higher oxides even though the sheet be discharged'from thefurnace at a rather high temperature.

In the performance of my said invention I employ any kind of continuousfurnace, such for example as that set forth in my Patent N 0. 1,623,469,April 5, 1927 or in my copending; application Serial No. 219,886, filedSeptember 16, 1927, and the important procedures are two in number, viz:that the heating of the steel is efiected in an atmosphere which islimitedly oxidizing to ferrous metal at the .temperature employed forthe first step of the process, namely, the heating above the criticalrange; and, second, the cooling below the critical range is effected inthis same limitedly oxidizing atmosphere, after which the-sheet can bedischarged into. a fully oxidizing atmosphere as quicklv as one pleases.

This limitedly oxidizing atmosphere is roduced, preferably, by burninghydrocar on fuel in a deficiency of oxygen so as to produce a mixtureof- H O, CO, and "a small amount of CO together of course with N, andsometimes with some free hydrogen or some free carbon or some freehydrogen and carbon though not always. The presence of the incompletelyoxidized fuel guards against the presence of free oxygen. The heat canalso be secured, if desired, by electrical means, but precautions areoften necessary to avoid dangerous explosions if electrical heat beemployed alone since the limitedly oxidizing atniosphere is generallyexplosive unless pro duced by incomplete combustion methods so that evenif electricity beemployed as the main source of the heat required (andit has some marked advantages in respect of distribution and control) itis best to employ at least fuel combustion to produce the necessaryatmosphere which must, to effect the purpose of my invention, be of anature to oxidize the ferrous metal but only to the limited degreementioned, which is secured when the oxygen required for the purpose isderived by the decomposition of oxygen-containing compounds in thefurnace atmosphere, sucl as CO This only occurs at the elevatedtemperature range and the oxide coating produced is non-progressive,self-protective, tough and so firmly adherent that bending of the sheetdoes not dislodge it. Furthermore it is stable against further oxidationif only the tempera ture of the sheet be slightly reduced before itsdischarge from the particular atmosphere. I havenot ascertained theexact temperature at which such film becomes insusceptible toatmospheric oxidation, but it is possible to discharge the sheets thuscoated from the furnace at a red heat, e. g. about 1400 F., or any lowertemperature Without any scaling. The surface of the sheet looks and actsmuch as does that of a heat-resisting chromium alloy, that is, itexhibits a silky, glossy sheen, often shown iridescent colors when hotbut nearly black when cold and Without any roughness.

In the drawing accompanying and forming a part of the application I haveshown a diagram of the time and temperature relations, showing also theincidence of my invention upon the usual process.

When the steel enters the furnace it is heated quickly from seasonaltemperature at A to a temperature at B which is above the criticaltemperature at which carbon is redissolved and grain-structure altered.It is'kept at this temperature for a. short time expressed in minutes orseconds depending upon the composition and thickness of the sheet Cafter which the temperature is dropped rather quickly to D where itfalls below the critical temperature. Thereafter the procedure followseither of two courses depending upon circumstances: if, as ispreferable, the sheets are still held inside the furnace (or otherchamber of restricted heat dissipating nature) the cooling curve followsthe path E. F. I.; if discharged immediately into the open air itfollows the path G. H. I.

My invention relates only to that part oi: the diagram comprised withinthe area K L BI N or K L M N, since it is only within this range thatthe lower oxide (which I regard as F e0 although without committingmyself to that composition) is formed. I do not 7 limit myself to anyfixed temperature for its formation since the composition of the steelprobably affects the same; neither do I l mit myself to any fixedcritical ten'iperature since the composition of the steel certainlyaffects the same. Neither do I assert an "elationship between thecritical temperature and the area of chemical action I have describedsince both of the same vary with differences of composition and may ormay not coincide. Accordingly I do not limit myself in any wiseexcepting as specifically recited in the follow ing claims which Idesire may be construed. each independently of limitations contained inother claims.

Having thus described my invention what I claim is: I

1. The improvement in the method of heat treating steel which consistsin heating the same quickly to a temperature above its criticaltemperature in a limitedly oxidizing atmosphere containing H O. CO, COand N but substantially devoid of free oxygen, and cooling such steel insuch atmosphere to a temperature which is both below the criticaltemperature and below any temperature at which such oxide of iron willreact with free atmospheric oxygen to form scale.

2. The improvement in the method of continuous heat treatment of steelsheets, strips, strands, rods, and wires which contains the steps ofconveying the same continuously through an atmosphere which ismaintained at a temperature above the critical temperature of the steeland containing a mixture of carbon monoxide and carbon dioxidesubstantially devoid of free oxygen, and reducing the temperature of thesteel to a degree below that at which the oxide so formed is reactive toatmospheric oxygen prior to removing the same from said firstatmosphere.

3. The improvement in the method of continuous heat-treatment of steelsheets which contains the steps of conveying the same continuouslythrough an atmosphere which is maintained at a temperature above thecritical temperature of the steel and which contains a mixture of carbonmonoxide and carbon dioxide, and thereafter reducing the temperature ofthe steel below about 1400 F. while still surrounded by an atmosphere ofthe composition described and before delivering it into the open air,whereby the decomposition of the carbon dioxide by the metal in thepresence of the carbon monoxide produces on the surface of the steel atough and adherent protective coating which prevents further oxidat ionof the metal upon its delivery into the open air substantially asdescribed.

In testimony whereof I hereunto aifix my signature.

FRANK A. FAHRENW'ALD.

